The present invention is directed to a process and apparatus for the regeneration of acids by pyrolysis of a spent acid liquor solution. More particularly, the invention is directed to a process and apparatus for the roasting of spent acids in a primary roasting furnace followed by a secondary roasting chamber.
Pickling acids, such as hydrochloric acid and nitric acid, are used in the treatment of iron and other metals. Typically, the spent acids are regenerated for reuse in various processes to minimize waste and problems associated with disposal.
Several acid regeneration processes pass the spent acid through a furnace to pyrolyze or roast the various components and to produce acid vapors. The vapors are passed through an absorption column counter-currently to an absorption liquid which is typically water. The regenerated acid is then withdrawn through the bottom of the column. The pyrolysis exhaust gases are withdrawn from the head space of the absorption column by an external exhaust fan which is supplied with a water spray to separate particulates remaining in the exhaust gas steam before discharging through a chimney.
Pickling acids of hydrochloric acid used to treat iron or steel produce numerous metal chlorides in the spent acid liquor. Many metal chlorides can be thermally converted into metal oxides and hydrogen chloride. The metal oxides then can be separated from the gaseous hydrogen chloride which can be recovered as hydrochloric acid for regenerating the pickling acids.
Typical processes of regenerating spent acids spray the aqueous acid solution into a heated reactor. The heated reactor heats the acid solution to vaporize the aqueous phase and acid compounds and produce particulates of metal salts that are contained in the spent acids as impurities and waste byproducts. The droplets of the spent acid solution and the particulates of the metal salts fall downwardly through the reactor where the particulates are removed. This process results in vary short retention time in the reactor, usually only a few seconds, which often produces metal oxide particulates having acid residues on the surface of the metal oxide particulates. The acid residues contaminate the metal oxides, thereby requiring further processing before the metal oxides can be used. The reactor temperature can be increased to completely separate the acid residues from the metal oxide particulates. Increasing the reactor temperature often overheats and decomposes a portion of the metal oxide particulates. Furthermore, the acid liquor is usually injected into the middle of the reactor so that the droplets in the middle are insufficiently heated, while the droplets on the outer areas of the reactor are overheated and partially decompose. In addition, increasing the reactor temperature results in a time lag before the desired roasting conditions are attained which can result in large amounts of acid residues being discharged with the metal oxides particulates. Other examples of processes for regenerating acids are disclosed in U.S. Pat. No. 4,049,788 and U.S. Pat. No. 5,149,515.
Accordingly, a continuing need exists in the industry for an improved process for the roasting of spent acid liquors.
The present invention is directed to a process and apparatus for regenerating spent acids, and particularly pickling acids. More particularly, the invention relates to a process and apparatus for the spray roasting of spent acid liquor using a roasting furnace having a secondary roasting chamber.
Accordingly, a primary object of the invention is to provide a process and apparatus for the complete and uniform roasting of a spent acid liquor and recovering the acid vapors.
Another object of the invention is to provide a process and apparatus for heating a spent acid liquor in a roasting furnace to separate a substantial portion of the acid vapors from the metal salts and transferring the metal salts to a secondary roasting chamber for further roasting of the metal salts.
A further object of the invention is to provide a process and apparatus for the spray roasting of spent pickling acids to produce a metal oxide of substantially uniform composition.
Still another object of the invention is to provide a process and apparatus for roasting a spent acid liquor to evaporate the water from the liquor and a secondary roasting chamber to heat and oxidize the metal salts.
The objects of the invention are basically attained by providing a process for regenerating spent acid liquor, comprising the steps of: feeding a spent acid liquor containing a metal salt into a primary roasting furnace and roasting the liquor to produce an acid vapor and a partially roasted metal salt; discharging the partially roasted metal salt from the roasting furnace to a secondary roasting chamber and completely roasting the metal salts to produce a metal oxide substantially free of acid residues; discharging the acid vapors from the primary roasting furnace and feeding to an absorption column; and feeding an absorption liquid to the absorption column to contact the acid vapors and produce a regenerated acid solution.
The objects of the invention are further attained by providing an apparatus for regenerating spent acid liquor, comprising: a primary roasting furnace having a spent acid liquor inlet, a heating device for heating spent acid liquor to evaporate a liquid portion and produce acid vapors and to produce a partially roasted metal salt, a first outlet for discharging acid vapors, and a second outlet for discharging partially roasted metal salt; a secondary roasting chamber having an inlet for receiving the partially roasted metal salt, a heating device for heating the secondary roasting chamber to a secondary roasting temperature to oxidize the metal salt substantially to a metal oxide and separate acid residues from the metal salt and an outlet for the acid vapors; and an absorption column for receiving acid vapors from the primary roasting furnace, and having an absorption liquid inlet for contacting the acid vapors and forming regenerated acid.
These and other advantages and salient features of the invention will become apparent from the annexed drawings and the detailed description of the invention.